For decades, entryway components including door jambs, brick mold, thresholds, and the like were made of wood because, among other reasons, wood is a dimensionally stable when exposed to extremes of temperature. That is, wood's coefficient of thermal expansion and contraction is extremely low. As a result, entryways made of wood can withstand even the most extreme temperature changes without warping due to expansion in hot temperatures and without exhibiting reversion (i.e. pulling away from adjacent structures) due to shrinkage in cold temperatures.
While wood entryway components indeed exhibit excellent dimensional stability when exposed to changes in temperature, wood also has inherent shortcomings that makes it less than optimal for fabrication of entryways. For instance, as wood supplies dwindle, wood becomes more and more expensive as a fabrication material, particularly in the high grades required for entryway systems. Perhaps more salient, however, is the fact that wood has a tendency to rot or otherwise deteriorate over time, particularly when exposed to moisture. This is particularly true for entryway components such as door jambs and brick mold because rain water tends to accumulate on the threshold at the bottoms of these components and seep into the end grain of the wood. The same is generally true for window frame components.
In recent years, many door and window manufacturers have attempted to develop door frame and window frame components fabricated from extruded plastic or plastic composites. Plastic offers advantages over wood because of its cost, durability, resistance to rot and decay, and other features. However, plastics, and particularly plastics such as PVC that are economically viable, have their own inherent shortcomings. For instance, most economically viable plastics that can be used to extrude door and window components have a heat distortion temperature (HDT) in the range of about 155° to 160° Fahrenheit. The significance of HDT is that once a plastic component is heated above this temperature, a change occurs in the molecular polymeric chains within the plastic. Specifically, the molecule chains tend to squeeze or bunch more closely together. The ultimate result is that when the temperature cools back below the HDT, the plastic component is permanently shortened, shrunk, or distorted. Obviously, such shrinkage is not tolerable in a door or window frame since it results in the distortion and gaps characteristic of reversion. In addition, plastics generally used for extruding door frame components have coefficients of thermal expansion that are significantly higher than wood. This can cause warping and buckling of a plastic door or window frame component in higher temperatures such as those commonly experienced in the South and Midwest.
Even with the forgoing shortcomings, extruded plastic door and window components made of economically viable plastics have proven themselves adequate for use in a significant number of applications in geographic regions where temperatures are moderate. In these applications, the temperature never rises high enough to cause warping due to thermal expansion and certainly does not rise above the HDT of the plastic, which can cause reversion in the plastic components. However, there is a significant number of applications and geographic regions where this is not the case. For example, when a glass storm door or storm window is installed on an entryway or window frame that is exposed to the sun, temperatures in the space between the storm door and entryway door can reach 185° Fahrenheit or higher, particularly where the entryway door is painted a dark heat absorbing color. Under these conditions, the thermal expansion of extruded plastic components made of PVC or other commonly used plastics causes these components to warp or bow. Further, since such temperatures are generally higher than the HDT of the plastic, reversion occurs in the plastic components, which results in distortion and the pulling away of the components from adjacent structures such as a threshold. Because of this, manufacturers of entryways with extruded plastic components warn against the use of storm doors with their entryways and often will not honor product warranties in the event of a storm door installation. Expansion induced warping and reversion also can be a problem in certain geographic regions such as in South Florida and the Midwest where temperatures in the summer can rise to over 110°. When extruded plastic door and window frame components are exposed to sun under these conditions, particularly if they are painted a dark color, warpage and reversion can occur.
Plastic materials do exist that have a higher HDT and a lower coefficient of thermal expansion than common plastics such as PVC. Such plastics include acrylonitrile-butadine-styrene (ABS), acrylonitrile-styrene-acrylate (ASA), and styrene-acrylonitrile (SAN) among others. The stability of such plastics can be increased even further by extruding them with a composite filler such as wood flour, ground rice hulls, or the like. Extruded door and window frame components made from such plastics or plastic composites indeed are very stable and can withstand temperature extremes with little or no warpage and reversion. Unfortunately, these plastics are dramatically more expensive than PVC and other common plastics, making door and window frames incorporating components made from them non-competitive in the marketplace.
Therefore, a need exists for an entryway (and a window for that matter) incorporating extruded plastic components that exhibit the dimensional and thermal stability of more expensive plastics and composites while at the same time being sufficiently economical so that the resulting products are competitive with existing wood and wood/plastic door and window frames. An entryway made with such components should remain dimensionally stable even when exposed to the very high temperatures generated when a storm door is installed and should withstand the extreme temperatures in particularly hot and cold regions of the country. It is to the provision of such an entryway and of dimensionally stable plastic components for use with entryways and windows that the present invention is primarily directed.